Maybe. Maybe not. You are probably one of the people on this board who 3 weeks ago said the same thing about Wisconsin.

Here are the most recent polls of California:

Survey USA: Trump 40%, Cruz 32%, Kasich 17%
USC: Trump 36%, Cruz 35%, Kasich 14%
NSON Opin: Trump 38%, Cruz 22%, Kasich 20%
PPIC: Trump 38%, Cruz 27%, Kasich 14%
SmithJohn: Trump 25%, Cruz 20%, Kasich 15%

Those are the only 5 polls conducted since March. According to Nate Silver's site, the weighted average of these polls is this:

35.3%- Trump
29.8%- Cruz
16.0%- Kasich

He gives more weight to the polls conducted more recently.

So while Trump leads the polls by about 5.5%, Silver also projects that right now, Cruz has a 60% chance of winning California. Why is that? Because Trump consistently proves that his pre-vote polling average is almost always equal to his ceiling. Bottom line is this: Most people who are going to vote for Trump have already made up their mind. Most people who are unsure at this moment, will end up voting for someone else if they vote.

The past few primaries have proven that a lot of people who say they are going to vote for Kasich end up voting for Cruz because they decide at the last minute not to waste their vote. In most of the recent primaries, Trump is getting the vast majority of the late-deciders, and he's taking a good chunk of Kasich's support at the last minute. This is why state-after-state, Cruz is ending up with 7-9% more of the vote in the actual election compared to his pre-vote polling average.

Silver projects that Cruz will win California, and the vote will look something like this:

41.1%- Cruz
36.7%- Trump
20.7%- Kasich

Of course this will change. That primary is not until June. But I think the longer this stretches out, the more it benefits Cruz. Kasich will continue to lose support (and donations) because he's simply not a factor. And every poll shows Cruz benefits more from Kasich supporters choosing another candidate.